You can do this using the "For" iterator. In this iterator you have to set a 'from' value and a 'to' value (here it depends on the value range of your DEM), and an incremental value (0.5). The tool you connect to 'Value' will execute for each value in the range you specify.
Add the Raster Calculator to your model, open it and build your expression by dragging the DEM and the 'Value' variable from the 'Layers and Variables' window to the expression window (see attached image). Don't forget to add '%Value%' to the name of your output, otherwise you will overwrite the same output raster n times. This will not work with negative values though, as hyphens can't be use to name datasets.